The present invention relates in general to a radio-tagged object location and tracking system of the type described in the U.S. Patent to Belcher et al, U.S. Pat. Nos. 5,920,287 and 5,995,046, (hereinafter referred to as the ""287 and ""046 patents, respectively), assigned to the assignee of the present application and the disclosures of which are incorporated herein, and is particularly directed to a reduction in the number of reader and associated RF channel processor components where the environment does not demand a high density geolocation infrastructure. Reducing the number of readers to relatively small number allows the use of a shared RF channel processor, to which the reader outputs are coupled in a time division multiplexed manner, so that the single RF channel processor can handle the output of each reader in a known, independent time frame.
The general architecture of the radio tagged object geolocation systems described in the above-referenced ""287 and ""046 Patents is diagrammatically shown in FIG. 1 as comprising a plurality of tag emission readers 10 that are installed at precisely geographically known and relatively unobtrusive locations in and/or around the perimeter of an asset management environment 12. In a typical environment, the readers may be spaced up to on the order 300 ft apart indoors and up to 750 ft apart outdoors.
The asset management environment contains a plurality of objects/assets 14, to which radio-containing xe2x80x98tagsxe2x80x99 16 are affixed. As a result of radio emissions from the tags 16, the locations of their associated objects 14 can be monitored on a continuous basis by the readers 10 and reported to an asset management data base 20. This data base is accessible by way of a computer workstation or personal computer 26.
In order for the system to locate and track the objects, each radio tag 16 repeatedly transmits or xe2x80x98blinksxe2x80x99 a short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of its associated object and auxiliary information that may be stored in tag memory. These short duration tag emissions are detected by the tag emission readers 10.
Coupled with each tag reader is an associated reader RF channel processor, which correlates the spread spectrum signals received from a tag with a set of spread spectrum reference signal patterns, in order to determine which spread spectrum signals received by that reader is a first-to-arrive spread spectrum signal burst as transmitted from a tag. Typically, RF transmissions from a tag are received at each reader with a delay of about one nanosecond for each foot of distance between them.
The identified first-to-arrive signals are then forwarded (via a coaxial cable plant) to an object geolocation processor, which performs time-of-arrival differentiation of the detected first-to-arrive transmissions, and locates (within a prescribed spatial resolution, e.g., on the order of ten feet) the tagged object of interest.
Because the lengths of cable plant installed between the readers"" associated RF channel processors and the differential time-of-arrival processing subsystem of a typical installation will vary among the various reader locations of the system infrastructure, they, as well as variations in environment, can be expected to introduce system timing errors (associated with the cable delays drifting due to weather or other effects such as age, humidity, temperature, physical stretching, and the like), resulting in geolocation errors.
Advantageously, the invention disclosed in the ""646 application effectively obviates this signal transport delay problem by placing one or more xe2x80x98referencexe2x80x99 tags, whose geolocations are fixed and precisely known within the environment containing the objects to be tracked, and executing a background calibration routine at a relatively low cycle rate, to process emissions from the reference tags. Pursuant to this routine, the calculated geolocations of the reference tags are compared with their actual locations, and any offset between the measured and actual geolocation values is then used to adjust the time delays of the various lengths of cable plant between the readers"" RF channel processors and the geolocation processor, so as to track out timing errors.
The above described geolocation system in which each reader has its own dedicated RF channel processor constitutes a relatively efficient use of resources for an environment having a large area and/or containing a relatively large number (e.g., thousands) of tags, where the readers and their associated RF channel processors are reused in up to four or more coverage zones. However, for a relatively small area environment having a lesser number (e.g., hundreds) of tags, the cost associated with installing a respective RF channel processor for each reader may be unacceptable to the user.
In accordance with the present invention, for an application that does not demand a high density geolocation infrastructure, the number of readers can be significantly reduced to only a few (up to about eight, as a non-limiting example). In addition, rather than dedicating a respective RF channel processor to each reader, plural reader outputs are time division multiplexed to a single shared RF channel processor, in a manner that allows the shared RF channel processor to receive and process each reader""s output in a known, independent time frame. This reduction in the number of reader and RF channel processor components enables the cost of the infrastructure of the geolocation system to be significantly reduced.
A first embodiment of the time division multiplexed signal transport network has a serial interconnectivity architecture, implemented as a daisy chain transport path among multiple readers and the shared RF channel processor. Respective segments of the daisy chain interconnect contain embedded delays that enable the shared RF channel processor to accommodate the output of each reader in a known, independent time frame.
In addition to whatever delay is inherent in the sections of cable, the transport delays may also include additional amounts of delay that provide isolation from delayed multi-path signals within a read interval. By periodically measuring the delay of each transport segment using the calibration mechanism detailed in the above-referenced ""646 application, timing errors in the times of occurrence of the first-to-arrive signals identified by the RF channel processor 50 can be effectively eliminated.
In order that the RF channel processor may determine which tag signal came from which reader, the RF signal emitted from a tag must be detected by that reader whose xe2x80x98time slotxe2x80x99 is the first time slot in the time slot sequence through which the reader outputs are time division multiplexed to the RF channel processor. If the geometry of the monitored environment is such that this cannot be effected for any reader at a perimeter location (such as an area corner), then an additional reader may be installed at a location that will ensure first detection, with the RF channel processor coupled directly to that reader.
A second embodiment of the time division multiplexed signal transport network has a parallel interconnectivity architecture, implemented as a star-configured set of transport paths among multiple readers and the shared RF channel processor. In the star configuration embodiment, the tag transmission readers are coupled via respective signal transport paths to a combiner, which is coupled to the RF channel processor. As in the serial embodiment, in addition to including whatever delay is inherent in their associated sections of cable, the transport delay stages of the star configuration embodiment impart additional delay, as necessary, to ensure isolation from delayed multi-path signals within a read interval. Also, these delays are preferably periodically measured using the calibration mechanism detailed in the ""646 application, to eliminate timing errors in the times of occurrence of the first-to-arrive signals identified by the RF channel processor.
Because emissions from a tag are asynchronous, there is no convenient way of knowing a priori when any of the readers will receive a transmission. To resolve this problem, the output of each reader may be modulated with a xe2x80x98signaturexe2x80x99 which associates that particular reader with the tag signal it receives. Non-limiting examples include amplitude and phase modulation. Of the two, phase modulation having a very low modulation index is more robust, and will not disrupt any existing modulations in use in the system. In addition, phase modulation may be readily detected in the RF channel processor by installing a straightforward software addition of demodulation code. Moreover, in the phase modulation approach, existing limiting and/or automatic gain control will not disrupt the signature signal. Also, the entire message length of a tag signal can be used to enhance detection probability.